The individual parts that are used to build an operating machine are each designed and built according to various part dimensions. As is generally known, a part dimension is a numerical value that defines the size, shape or location of a feature. In many instances, the parts that comprise a machine are each described by one or more two-dimensional drawings. These drawings typically include part geometry, nominal part dimensions, part tolerances, and may additionally include various other part characteristics such as, for example, the material from which the part is to be constructed, and various mechanical properties such as tensile strength or hardness. The tolerance values, as is generally known, specify an acceptable range of variation from the nominal part dimension. A single part can be governed by a plurality of tolerances, each tolerance related to a different geometric feature of the part. For example, the drawings for an aircraft engine gas turbine blade may include as many as four hundred dimension tolerances.
Parts are preferably manufactured to conform to the associated drawings, considering both the nominal dimensions and the tolerances associated therewith. If a part is manufactured such that one or more of its actual dimensions is outside the tolerance range specified in the drawing, it may not be suitable for use in the machine. For example, it may not properly mate or interface with another part or may physically interfere with the operation of another part.
As the individual parts are assembled to form the machine, radial and axial gaps between parts are preferably maintained within design limits. A nominal gap distance represents the desired gap opening. A gap tolerance indicates a range of acceptable variations from the nominal gap distance. For example, in a gas turbine engine, combustion gases impinge upon a plurality of blades carried by a spinning rotor enclosed within a stationary shroud. Maintaining a specified gap (as defined by the nominal running gap distance and a tolerance range associated with the gap distance) between a tip of each rotor blade and the shroud ensures proper and efficient operation of the engine. Thus, dimension tolerance stack-up analyses are preferably conducted to ensure appropriate gaps are maintained.
Dimension tolerance stack-up analysis is a process of using given machine part dimensions and part tolerances to predict the dimension and tolerance of an assembly dimension between two mating or adjoining parts, e.g., to predict the nominal assembly dimension and tolerance of a machine gap. Typically, the gap stack-up analysis is performed manually, and includes identifying at least two parts and the dimensions of those parts that create the gap. These part dimensions form a stack path beginning at one gap surface, traversing through serial part interfaces until reaching the opposing gap surface. The part dimensions and part dimension tolerances associated with each such part are then combined to yield the gap nominal dimension and the gap tolerance. It may thus be appreciated that this analysis can be a rather tedious, time consuming, and potentially costly undertaking.
In addition, the operating efficiency of a gas turbine is at least partially dependent upon the radial clearance or gap between rotor blade tips and the shroud. If the clearance between the rotor blade tips and the surrounding shroud is too large, additional flow may leak through the gap between the rotor blade tips and the surrounding shroud, decreasing the turbine's efficiency. Conversely, if the clearance is too small, the rotor blade tips may strike the surrounding shroud during certain turbine operating conditions. To facilitate optimizing the turbine efficiency, the clearance is preferably adjusted to enhance steady-state performance while maintaining an adequate clearance margin as the turbine accelerates and decelerates. Moreover, a cold clearance that is initially relatively tight, can result in excessive regenerative rubs. Over time, continued rubs may cause loss of material and/or a blunt or mushroomed seal tooth which may change the flow characteristics and adversely affect the performance of the turbine. A balanced design may provide tight average operating clearances as well as facilitate avoiding rubs during transients and operating at off-design conditions.
Turbine radial clearances may change during periods of acceleration or deceleration due to changing centrifugal force generated in the rotor, and/or due to relative thermal growth between the rotating rotor and stationary shroud. During periods of differential centrifugal and thermal growth, clearance changes may result in rubbing of the moving blade tips against the stationary shroud. Such an increase in blade tip clearance results in efficiency loss. Since components of turbines and other rotating machines are, in many instances, made of different materials with different thicknesses, such components exhibit different rates of thermal growth from a cold startup condition to steady state operating condition and during transient operating conditions. Such differences make calculating gap clearances difficult and time consuming.
Hence, there is a need for a system and method that can readily determine at least radial gaps during both non-operational and operational conditions in a rotating machine. The present invention addresses at least this need.